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His doctor drove him over the border. It was quicker that way: If the man donated in Switzerland, his blood would be delayed while paperwork was filled out and authorizations sought.

The nurse in Annemasse, France, could tell from the label on the blood bag destined for Paris that this blood was pretty unusual. But when she read the details closely, her eyes widened. Surely it was impossible for this man seated beside her to be alive, let alone apparently healthy?

Thomas smiled to himself. Very few people in the world knew his blood type did—could—exist. And even fewer shared it. In 50 years, researchers have turned up only 40 or so other people on the planet with the same precious, life-saving blood in their veins.

Red blood cells carry oxygen to all the cells and tissues in our body. If we lose a lot of blood in surgery or an accident, we need more of it—fast. Hence the hundreds of millions of people flowing through blood donation centers across the world, and the thousands of vehicles transporting bags of blood to processing centers and hospitals.

It would be straightforward if we all had the same blood. But we don’t. On the surface of every one of our red blood cells, we have up to 342 antigens—molecules capable of triggering the production of specialized proteins called antibodies. It is the presence or absence of particular antigens that determines someone’s blood type.

Some 160 of the 342 blood group antigens are “high-prevalence,” which means that they are found on the red blood cells of most people. If you lack an antigen that 99 percent of people in the world are positive for, then your blood is considered rare. If you lack one that 99.99 percent of people are positive for, then you have very rare blood.

If a particular high-prevalence antigen is missing from your red blood cells, then you are “negative” for that blood group. If you receive blood from a “positive” donor, then your own antibodies may react with the incompatible donor blood cells, triggering a further response from the immune system. These transfusion reactions can be lethal.

Because so few people have it, by definition, rare blood is hardly ever needed. But when it is, finding a donor and getting the blood to a patient in crisis can become a desperate race against the clock. It will almost certainly involve a convoluted international network of people working invisibly behind the bustle of “ordinary” blood donation to track down a donor in one country and fly a bag of their blood to another.

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Forty years ago, when 10-year-old Thomas went into the University Hospital of Geneva with a routine childhood infection, his blood test revealed something very curious: He appeared to be missing an entire blood-group system.

There are 35 blood-group systems, organized according to the genes that carry the information to produce the antigens within each system. The majority of the 342 blood-group antigens belong to one of these systems. The Rh system (formerly known as “Rhesus”) is the largest, containing 61 antigens.

The most important of these Rh antigens, the D antigen, is quite often missing in Caucasians, of whom around 15 percent are Rh D-negative (more commonly, though inaccurately, known as Rh-negative blood). But Thomas seemed to be lacking all the Rh antigens. If this suspicion proved correct, it would make his blood type Rh_null—one of the rarest in the world, and a phenomenal discovery for the hospital haematologists.

Rh_null blood was first described in 1961, in an Aboriginal Australian woman. Until then, doctors had assumed that an embryo missing all Rh blood-cell antigens would not survive, let alone grow into a normal, thriving adult. By 2010, nearly five decades later, some 43 people with Rh_null blood had been reported worldwide.

Hardly able to believe what she was seeing, Dr Marie-José Stelling, then head of the hematology and immunohematology laboratory at the University Hospital of Geneva, sent Thomas’ blood for analysis in Amsterdam and then in Paris. The results confirmed her findings: Thomas had Rh_null blood. And with that, he had instantly become infinitely precious to medicine and science.

Researchers seeking to unravel the mysteries of the physiological role of the intriguingly complex Rh system are keen to get hold of Rh_null blood, as it offers the perfect “knockout” system. Rare negative blood is so sought after for research that even though all samples stored in blood banks are anonymized, there have been cases where scientists have tried to track down and approach individual donors directly to ask for blood.

And because Rh_null blood can be considered “universal” blood for anyone with rare blood types within the Rh system, its life-saving capability is enormous. As such, it’s also highly prized by doctors—although it will be given to patients only in extreme circumstances, and after very careful consideration, because it may be nigh on impossible to replace. “It’s the golden blood,” says Dr. Thierry Peyrard, the current Director of the National Immunohematology Reference Laboratory in Paris.

Blood groups are inherited, and Rh_null is known to run in families. So the next step for the haematologists in Geneva was to test Thomas’ family in the hope of finding another source, particularly as Thomas wouldn’t be able to donate until he turned 18. Things looked even more hopeful when it turned out Thomas’ grandfathers were third-degree cousins. But the tests showed Thomas’ Rh_null blood was due to two completely different random mutations on both sides. Pure chance, twice over, in the face of vanishingly small odds.